1. Field of the Invention
The present invention relates to an exhaust gas purification system for an internal combustion engine provided with a zeolite-type NOx reduction catalyst in an exhaust conduit of the engine wherein a high NOx purification rate of the catalyst can be obtained even after the catalyst has been degraded.
2. Description of the Prior Art
Carbon dioxide (CO.sub.2) exhausted from automobile engines is desired to be reduced for environmental protection, and fuel combustion of engines at lean air-fuel ratios (lean burn) is one solution therefor. However, since a conventional three-way catalyst cannot reduce nitrogen oxides (NOx) included in the exhaust gas from the lean burn engine, it is a problem how to decrease the amount of NOx exhausted from the engine to the environment.
As a catalyst capable of reducing NOx under oxidizing gas conditions (exhaust gas conditions of the lean burn engine), a catalyst constructed of zeolite carrying transition metals and reducing NOx in the presence of hydrocarbons (HC) is disclosed in, for example, Japanese Patent Publications HEI 1-130735 and HEI 1-135541.
However, with the zeolite catalyst, there is a problem that the catalyst tends to undergo thermal degradation which makes it impossible to utilize the catalyst effectively.